The meaning and purpose of the volume field of the dockerfile file

[cai-hd]

When I run the container using the mysql:5.7.32 image, like this:

docker run -e MYSQL_ROOT_PASSWORD=root -d mysql:5.7.32

When I run a new container every time. A new volume will be generated in the /var/lib/docker/volume directory,
I understand this because there is a data volume declared in the dockerfile file.

But I have two questions.

1. What is the purpose of this anonymous volume ？
2. When I started the container, the volume generated was about 200MB. I deleted the container, but the volume still exists and I need to delete it manually. What is the meaning of this design?

[wglambert]

There's a previous discussion on this over at #255